Airborne wind energy system with enhanced power transfer

ABSTRACT

An improved wind power device for wind energy conversion or vehicle propulsion. Among many possibilities contemplated, the device may have a moving sail with tethered wings ( 101 ), moving in elliptical trajectory, utilize separate sheave ( 503 ) and cable drum ( 505 ), use a block and tackle ( 411 ), attached to the tether and utilize a cable having a flexible jacket with aerodynamically streamlined cross section ( 603 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a continuation of U.S. patent application Ser. No.14/266,765, filed on 30 Apr. 2014, which is a continuation of PCTApplication No. PCT/US12/67143, filed 29 Nov. 2012, which claims thebenefit of U.S. Provisional Applications No. 61/566,681, filed 4 Dec.2011, No. 61/577,329, filed 19 Dec. 2011, No. 61/621,535, filed 8 Apr.2012, No. 61/621,593, filed 9 Apr. 2012, No. 61/624,470, filed 16 Apr.2012, No. 61/662,476, filed 21 Jun. 2012 by the same inventor as herein.All of the foregoing applications are hereby incorporated herein intheir entirety.

BACKGROUND OF THE INVENTION

This invention is generally directed to wind power utilizing systems andmethods, using airborne wings or sails.

Recently, a novel approach to wind power utilization has appeared. Acomputer controlled kite, flying crosswind, harnesses power of the wind,which is further converted into electric energy or into propulsion of aship. One example of former is U.S. Pat. No. 8,080,889 by Ippolito et al(assigned to KiteGen). One example of later is U.S. Pat. No. 7,672,761by Wrage (assigned to SkySails). The common part is that the kite movescross wind with high speed in so-called ‘figure 8’ trajectory. Thetether of the kite also moves crosswind and experiences very large drag,which can exceed the drag of the kite itself. This drag wastes energyand limits possible length of the tether.

The crosswind flying airborne wing develops high lift forces. In theelectricity generating applications, the speed of the tether,transferring this lift to the rotor of the electric generator, isrelatively low (typically about ⅓ of the wind speed), resulting inrelatively low power output for the force. This issue is furtherexacerbated by unwinding the tether from a tether drum, and using thesame drum as a rotational element, converting linear motion of the cableinto rotational motion. The drum is wide, and its width furtherincreases when the tether's thickness increases. Consequently, drum'sRPM is low and it requires an expensive gearbox with high input torqueand large conversion ratio in order to achieve 1,500-1,800 RPM, requiredby most conventional electric generators.

One attempt to solve the problem of high cable drag is U.S. patentapplication Ser. No. 12/154,685 Faired Tether for Wind Power GenerationSystems by Griffith et al. Unfortunately, the tether in that applicationis prohibitively expensive or inefficient.

This invention is directed to solving these problems and more.

SUMMARY OF THE INVENTION

One embodiment of the invention is a moving sail for use in systems,utilizing wind power, comprising at least two airborne wings; a platformat the ground level; a pulled element attached to the platform; atether, connecting the wings to the pulled element; an anti-twistdevice, preventing twisting of the tether by motion of the wings; andthe wings move under influence of the wind in the same clockwise orcounter clockwise direction, if viewed from the platform, and the motionof the wings has substantial cross wind component. Another embodiment ofthe invention is a moving sail for use in systems, utilizing wind power,comprising two or more two airborne wings; a platform at the groundlevel with a pulled element attached to it; an airborne attachmentdevice, having two sides, allowed to freely rotate one relative toanother; each wings is attached to one side of the attachment device bya flexible cable; and a tether, attached to another side of theattachment device and to a pulled element of said platform; the wingsmove under influence of the wind in the same clockwise or counterclockwise direction, if viewed from the platform, and the motion of thewings has substantial cross wind component.

Related method of utilizing wind power, comprising steps of providing atleast two airborne wings, attached to a tether by cables; providing aplatform having an element, pulled by the tether at the ground level andcontrolling the wings to move at least partially cross wind, in the sameclockwise or counter clockwise direction relative to the platform formultiple loops, while preventing twisting of the tether.

Another aspect of the invention is a device for conversion of windenergy into electric energy, comprising an airborne wing or sail, movingunder power of wind; a cable, attached to this wing or sail; a block andtackle system, attached to the cable; a ground level platform with arotational element (like a sheave, a pulley or a sprocket) on it,coupled to the block and tackle system and an electric generator with arotor rotationally connected to the rotational element.

Another aspect of the invention is a device for conversion of windenergy into electric energy, comprising an airborne wing or sail, movingunder power of wind; a cable, attached to this wing or sail; a groundlevel platform with a rotational element (like a sheave, a pulley or asprocket) in contact with the cable; an electrical generator, having arotor rotationally connected to the rotational element; and means forholding excess of said cable (like a cable drum). The cable may comprisetwo dissimilar sections, a top section and a bottom section, and onlythe bottom section is exposed to said rotational element.

Related method of converting linear motion of a cable into rotationalmotion in a wind energy conversion device, comprising steps of providingan airborne wing; a cable, coupled to the airborne wing; a rotationalelement coupled with a rotor of an electric generator; coupling thecable with the rotational element; storing excess of the cableseparately from the rotational element. Further, the top part of thecable, which normally does not come in contact with the rotationalelement, may have round or streamlined section; and the bottom part ofthe cable, which does come in contact with the rotational element, mayhave flat or flattened section. An electronic control system may beutilized to control the electrical generator and/or the rotationalelement and/or to synchronize reel on/reel off of said cable with themotion of the airborne wing.

Another aspect of the invention is a cable with aerodynamicallystreamlined profile, comprising a load bearing core and a flexiblejacket having aerodynamically streamlined profile around the core,placed in such way that the center of aerodynamic pressure on the jacketis behind the center of the core, when the profile of the cable is notoriented straight into the relative airflow.

A method of manufacturing a cable with aerodynamically streamlinedprofile, comprising steps of providing at least one core cable made of amaterial with high tensile strength and wrapping a flexible jacket,having aerodynamically streamlined profile, around it.

Various objects, features, aspects, and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention, along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. The illustrationsomit details not necessary for understanding of the invention, orobvious to one skilled in the art, and show parts out of proportion forclarity. In such drawings:

FIG. 1 is a schematic view of a vehicle propulsion system with a dynamicsail according to one aspect of the present invention

FIG. 2 is a schematic view of a rigid wing when used in the dynamic sail

FIG. 3 is a schematic view of a flexible wing when used in the dynamicsail

FIG. 4 is a schematic view of a wind energy conversion system accordingto one aspect of the present invention

FIG. 5 is a perspective view of a wind energy conversion system with aseparate pulley or sprocket and a cable drum

FIG. 6A is a sectional view of one form of a aerodynamically streamlinedcable

FIG. 6B is a perspective view of one form of the aerodynamicallystreamlined cable

FIG. 7 is a sectional view of another form of the aerodynamicallystreamlined cable

FIG. 8 is a sectional view of one more form of the aerodynamicallystreamlined cable

FIG. 9 is a sectional view of yet another form of the aerodynamicallystreamlined cable

FIG. 10 is a perspective view of another form of the aerodynamicallystreamlined cable

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless stated otherwise, term “cable” here includes usual mechanicalcables, ropes and lines of any form and material. It also encompassesbelts, including perforated belts, flat belts, round belts, toothedbelts, ribbed belts, grooved belts and V-belts. A tether is a kind of acable, lower end of which is attached to an object on the ground level.

FIG. 1 shows one embodiment of the invention, in which a system ofairborne wings utilizes power of the wind to pull a ship. Thisembodiment comprises a pair of wings 101, attached by cables 102 to ananti twist device 103. Anti twist device 103 is attached to a ship 110by a tether (or another cable) 105. A control system 104 is provided.Ship 110 has a hull 106 and a rudder 107. Anti twist device 103 isprovided in order to allow circular motion of wings 101. Anti twistdevice 103 comprises a top part 109 and a bottom part 108 with a ballbearing between them, allowing unlimited rotation of top part 109relative to bottom part 108. Optionally, it can be provided with its owndirection sensor (gyroscopic, magnetic or GPS) and a servomotor,compensating remaining twisting moment.

Wings 101 move cross wind in a circle in the same direction (clockwiseor anti clockwise, when viewed from ship 110) under power of wind forlong time. The circle lies in a plane—the plane of rotation. In FIG. 1,the plane of rotation is inclined about 45 degrees to the horizon. Theaerodynamic forces (mostly aerodynamic lift) act on wings 101 and aretransferred to anti twist device 103. There, force components, parallelto the plane of rotation, compensate each other. The remaining forcecomponent, normal to the plane of rotation, pulls tether 105, whichpulls ship 110. The projection of tether 105 on horizontal plane doesnot usually match direction of the desired motion. Rudder 107 and hull106 compensate sideways component of the pull force. Control system 104selects direction of the tether to maximize the component of the pullforce, matching the desired direction of ship motion (tractivecomponent). Control system 104 can vary angle of attack of the wingsdepending on the wind condition and desired pull, and angle ofinclination. Angle of inclination of the plane of rotation to thehorizon can vary in wide range, from 30 to 85 degrees.

This system can be used either as an auxiliary propulsion system, as amain propulsion system with an auxiliary engine or, on a small boat, asa sole propulsion system. This system cannot pull ship 110 directlyupwind. If upwind motion is desired, the system should be eitherdepowered (wings are let to move with a minimum lift, required to keepwings 101 in the air) and a conventional engine used, or the ship shouldbe tacking.

More than two wings can move in the same plane of rotation. Multipleanti twist devices 103 can be connected with long tethers on top of eachother, with a system of wings connected to each anti twist device 103and moving in parallel planes.

Lateral axis of wings 101 have slight inclination to the plane ofrotation. Wings 101 are cambered. In one particular embodiment, thelateral axis of the wings 101 are inclined 100 to the plane of theirrotation, and the angle of attack is 3°. The angles change, depending onstrength of the wind and the required force. In another example,longitudinal axis of each wing 101 has constant angle 5° to the plane ofrotation, and angle of attack changes with the position of the wing inthe circle.

The system of wings 101 plays role of a conventional sail, with a bigadvantage: fast crosswind motion of the wings allows to develop force,many (possibly hundreds times) bigger than wind pressure on static sailsof the same size. Another advantage is that it can catch stronger andmore regular wings at the altitude above the sea level. Further, tether105 does not exhibit significant motion and does not create significantdrag. Also, circular motion of wings 101 requires lower centrifugalacceleration (compared with figure 8 motion).

Wing 101 can be any of the following: a rigid wing, like planes, glidersor ground based wind turbines have; a flexible wing; a soft wing; aninflatable wing; an inflatable wing, inflated by the ram air, enteringit through holes; a kite wing; a paraglider wing; a wing, using softmaterials, spread over a rigid frame or cables; a wing made of elasticfabric, receiving airfoil form from relative air flow; and/or a mixedwing, using different construction techniques in different parts of thewing.

Wing 101 can be made of various materials, including carbon fiber,fiberglass, wood, aluminum, aramids, para-aramids, polyester, highmolecular weight polyethylene, nylon and others. Wing 101 may havewingtips to decrease turbulence and noise. Wing 101 has stabilizationand control surfaces and their actuators and possibly its own controlsystem. An example of a rigid wing is shown in FIG. 2. It compriseshorizontal stabilizers 201, rudders 202, a vertical stabilizer 203 andan elevator 204 on a double boom 206, spoilers 205 and a control system207. An example of a kite wing is shown in FIG. 3. It comprises flexibleinflatable canopy 301, 4 combined control and suspension cables 302 anda control device 303. In this form, position of the wing relative to thewind and to the horizon is controlled by dynamically changing thelengths of cables 302. Wing 101 can be aerodynamically unstable and itsstability can be assured by frequent application of corrective forces.

Control system 104 comprises a central processor or a microcontroller,actuators, sensors and communication means for communicating with thecontrol elements of wings 101. Preferable communication means is awireless network, although optical or copper wires, going through cables102 and tether 105 can be used too. The sensors may include ananemometer, barometer, radar, hygrometer, thermometer, GPS, cabletension meter, RPM meter, cameras for observing the wings and other.

FIG. 4 shows another embodiment of the invention. A pair of wings 101connected by cables 102 to anti twist device 103 are is placed in theair and are flying cross wind with a speed, exceeding speed of the wind.Wings 101 can have a high L/D ratio, and move with speed is 4-20 timeshigher, than the speed of the wind. A cable 401 is attached to antitwist device 103 at one end and to a sheave 403 at another end. A groundplatform 410 is installed on the ground, or slightly above the ground.An electric generator 408, having a rotor and a stator, is installed onthe platform. A pulley 407 is rotationally connected to the rotor ofelectric generator 408. The connection can be via a gearbox, or pulleycan be co-axial with the rotor, or another way of mechanicaltransmission can be utilized. Platform 410 can be able to rotate inhorizontal plane (yaw) to order accommodate changes in direction of windand movement of the wing. A sheave 405 is installed on platform 410. Abelt 406 is attached by its one end to platform 410, goes around anothersheave 403, then around sheave 405, then around one more sheave 404, andcomes into contact with pulley 407. Belt 406 wraps around pulley 407 ata number of times, necessary to avoid slippage (this number can bebetween 0.25 and 20, depending on used materials, cable form and otherconditions). Remaining part of cable 406 is wound around a spool 409.

Usual mechanical cables, ropes and lines of many forms and materials canbe used for belt 406. Also, various belts, including perforated belts,flat belts, round belts, toothed belts, ribbed belts, grooved belts,V-belts and other can be used for belt 406. Control system 412 isprovided.

Operation of this embodiment is controlled by control system 412.Operation comprises two phases: the active phase and the passive phase.The active phase starts when anti twist device 103 is in a position,closest to platform 410, sheave 403 is closest to sheave 405, and almostall of cable 406 is wound on spool 409. In a coordinate system, movingwith anti twist device 103, wings 101 move in the rotation plane.Relative to platform 410, wings 101 move in ascending downwind spiralwith constant radius, getting away from the platform. Aerodynamic liftof wings 101 pulls cable 401, which pulls sheave 403. Belt 406 is pulledup, unwinding from spool 409 and rotating pulley 407, which rotates therotor of electrical generator 408, which produces electric energy. Whenall cable 406 is unwound from spool 409, an electric motor or some othermeans stop spool 409 and start rotating it in the opposite direction,winding cable 406 back on. Winding cable 406 pulls in sheave 403 andextension cable 102. Wings 101 stop flying cross wind and are commandedby control system 412 to fly in general direction of platform 410,creating minimum resistance, only to keep cables 102 and 103 stretched.In the end of passive phase, the device returns into the initialposition, and a new active phase starts. The passive phase is muchshorter than active phase and consumes very little energy.

Optional block and tackle 411 is employed to mechanical disadvantage. Itis used here for two purposes:

a) increase velocity of belt 406 in contact with the pulley, thusdecreasing forces, acting on the pulley and other mechanisms, connectedto it, for the same power;

b) decrease tension of belt 406, thus allowing to decrease its thicknessand, consequently, diameter of pulley 407, while increasing durabilityof the belt.

Increasing velocity of cable in contact with pulley 407 and decreasingdiameter of pulley 407 allow to increase angular speed of pulley'srotation. A gearbox may still be required, but less expensive one thanwithout use of block and tackle 411. FIG. 4 shows block and tacklesystem with mechanical disadvantage ratio of 4 (i.e., velocity of thecable near the pulley, attached to the rotor, is 4× higher than thespeed, with which distance between wing 101 and generator 408increases). By changing number of the sheaves, it is possible to changemechanical disadvantage ratio from 2 to 20. Block and tackle system 411or its analogies (a differential pulley, Z-drag line, Spanish bartonsetc.) can be used in any wind energy conversion system with airborneblades, where the motion transfer is performed by a cable.Alternatively, belt 406 can be connected directly to cable 401. If belt406 is perforated, a matching sprocket can be used instead of pulley407.

An example system with cross wind wing trajectory, in which anti twistdevice 103 is moving away from platform 410 with an average speed 2 m/s,pulley 407 has diameter 0.25 m and block and tackle provides 10×mechanical disadvantage has 1,500 RPM on pulley, sufficient for almostevery 50 Hz electric generator without gearbox.

It should be noted, that in the active phase cable 401 moves steadily inthe direction of its length and neither it nor block and tackle system411 experience significant sideways motion (thus saving power losses dueto air resistance and excessive wear of cable 406). Different strategiesfor control of wings 101 can be utilized by control system 412 in theactive phase. One strategy is to attempt to keep wing's angle of attackin the air constant and low. Another strategy is to attempt to keepconstant the wing's angle to the wings' plane of rotation. Thesecontrols actions can be combined with cyclical changing angle of thelateral axis of the wings to their plane of rotation (over each 360degrees rotation cycle). Anti twist device 103 prevents twisting ofcable 401.

FIG. 5 shows another aspect of the invention—an enhanced mechanism forconversion of linear motion of a cable into rotational motion of a rotorof electric generator. In one embodiment, this aspect of the inventioncomprises at least one wing 501, moving in the air under power of windand pulling a cable 502. On the ground, there is an electric generator504, comprising a rotor and a stator. A pulley (or a sheave, or asprocket for a perforated belt) 503 is rotationally attached to therotor of generator 504. Optionally, it can be attached through a gearbox(not shown on the picture). Cable 502 is wrapped around pulley 503 atleast pre-defined number of turns. Pre-defined number of turns can befractional and is usually small, typically between 0.5 and 20. Afterwrapping around pulley 503, cable 502 is wound around a spool 505, andcable's end is attached to it. Means are provided to wind and unwindcable 502 accurately and to maintain pre-defined force on cable 503 indirection from pulley 503 to spool 505 in order to ensure sufficientfriction. In FIG. 5, these means are a small electrical motor 506,attached to spool 505, and a ball screw 507 with associated smallelectric motor 508. Further, an optional section of cable 509 of adifferent kind can be inserted between wing 501 and cable 502. Forexample, cable 509 can be a standard round cable, while cable 502 can bea perforated belt. If cable 502 is perforated belt, a sprocket is usedinstead of pulley 503.

Operation of the system consists of two phases—the active phase and thepassive phase. In the active phase, wing 501 is moving away from pulley503, while rotating the rotor of generator 504, which generateselectricity. It should be noted, that trajectories of the wing maydiffer as long as the cable length between wing 501 and pulley 503increases. In the passive phase, wing 501 moves toward pulley 503, whilecable 502 is winding on spool 505. Electrical energy is not generated inthe passive phase, other way around—small amount of electrical energymay be consumed. The arrows on the picture show direction of movement ofcable 502, spool 505 and direction of rotation of pulley 503 and spool505 in the active phase. In the passive phase these directions areopposite.

In the beginning of the active phase, spool 505 is in its left mostposition, wing 501 is in the position, closest to pulley 503 and most ofcable 502 is wound on spool 505. In the active phase wing 501 is movingaway, pulling cable 502. Cable 502 rotates pulley 503 and unwinds fromspool 505. Small motor 506 resists rotation of spool 505, creating aforce on the segment II of cable 502, opposite to direction of cablemovement. Force, acting on segment II, is much smaller than force ofwing's pull, acting on segment I, so that cable unwinds, while rotatingpulley 503 without slippage. As cable 502 unwinds from spool 505, spool505 moves toward right on ball screw 507, pushed by motor 508. Spool 505moves to right with such speed that unwinding cable remains aligned withpulley 503. In the end of active phase, spool 505 is in its right mostposition, wing 501 is in the furthest position from pulley 503 and onlyfew wraps of cable 502 remain on spool 505. Then passive phase starts.In this phase, motor 506 rotates pulley 506 in the opposite direction,winding cable 502 on pulley 505, while motor 508 moves spool 505 to theleft. In the end of the passive phase, positions of the parts of thesystem correspond to the positions in the beginning of the active phase.

The benefits of this embodiment are due to the fact, that forces, actingon segment II of cable 502 are much smaller than forces, acting onsegment I of cable 502. The ratio is determined by belt frictionequation:

T_(load)=T_(hold)e^(μφ)

where T_(load) the force on segment I, T_(hold) is the force on segmentII, mu is the coefficient of friction and phi is the angle of cablewrapping. It is desirable to have higher ratio of forces. In many casesratio 20:1 is sufficient (i.e., force on segment II is 5% of force onsegment I). To achieve such ratio with a high friction ribbed belt overribbed sheave(mu=0.9), only half turn of the cable is required. Toachieve such ratio with Dyneema over smooth steel (mu=0.05), full 20turns are required. In any case, the force should be sufficient toprevent slippage. Since force, acting on segment II of cable 502 isrelatively small and we do not care about angular velocity of spool 505,spool 505 can be wide and long and cable 502 can be laid in multiplelayers on it.

Among advantages of this aspect and embodiment: very long cables andvery long cable motion are allowed (up to tens of kilometers) due tolarge capacity of spool 505; cables and belts, withstanding high tensionare allowed due to large capacity of spool 505; cable fatigue isdecreased due to large diameter of spool 505; flat cables and belts canbe used; rotational velocity of pulley 503 can be increased for the samelinear velocity of cable 502 by decreasing diameter of pulley 503;higher power output at lower cost is achieved.

As a variation of this embodiment, a second wing can be used instead ofspool 505, creating resistance in segment II of cable 502 in its activephase, and pulling cable 502 in the passive phase of the first wing.Also, drag based wind capturing devices can be used instead of wings inthis embodiment.

Another aspect of the invention is a streamlined cable and method of itsmanufacturing, combining high strength with low drag in cross flow andlow disturbance of laminar air flow. The high drag of a usual cable iscaused by it being round in section. This causes disturbance of air flowin the area behind the cable. Thus, aerodynamic cable should have asection in the form of a streamlined body.

FIG. 6A shows one embodiment of such aerodynamic cable in section. Itconsists of an off-the-shelf load bearing rope 601 inside of astreamlined jacket or coating 603, with the remaining space occupied bypolyethylene foam 602. Rope 601 is placed in the widest place insidejacket 603 (or jacket 603 is wrapped around rope 601). In some variantsof this embodiments, especially when the rope diameter is small,polyethylene foam 602 is replaced by air. Also, electrical or opticalwires can pass in the space between core 601 and jacket 603. Other lightweight flexible material can be used instead of polyethylene foam. Rope601 is made of aramid (including poly aramid or meta aramid) fibers,ultra high molecular weight polyethylene fibers, carbon fibers oranother strong material. jacket 603 is made of nylon or other material,sufficiently durable and flexible. Material of jacket 603 should also besmooth from outside, resistant to water and ultra violet radiation, orhave coating with these properties. When the aerodynamic cable isattached to another cable or surface, jacket 603 and foam 602 areremoved at the end, and rope 601 is attached as usual.

FIG. 6B shows perspective view of the aerodynamic cable according tothis embodiment, with rope 601 exposed at the end. In most cases, thecable needs to orient itself correctly in the air stream. The cable inthis embodiment does orient itself in the air stream, when it isattached by its end and sufficiently long. In some variations,additional strips 604 are attached to the rear end of the cable atconstant intervals, to provide additional stabilization in the air flow.Strips 604 can be rigid or flexible.

FIG. 7 shows another embodiment of the aerodynamic cable in section. Itcomprises multiple ropes 601, possibly of different diameters. In thisembodiment, ropes or yarns 601 can fill part or whole space inside ofjacket 603. FIG. 8 shows another embodiment of such aerodynamic cable insection. It uses one or more sideways compressed ropes 601. As theresult of asymmetrical compression, rope 601 should be wider in thedirection of the airfoil axis and narrower in the perpendiculardirection. FIG. 9 shows another embodiment of such streamlined cable, inwhich sectional form is not airfoil, but round in the forward part andangular in the rear part. FIG. 10 shows another embodiment, in which asmall section of jacket 603 is removed at equal intervals (for example,each one hundred diameters of rope 601). It is done for betterflexibility and self orientation in the air flow.

These embodiments of invention will have aerodynamic drag many times(5×-50×) lower, than round cable of the same strength and weight, due tolower form drag coefficient in all embodiments and lower cross sectionin some embodiments for the same strength. The applications of thestreamlined cable are in the airborne wind energy conversion devices,tethered aircraft, suspension cables for airplanes and kites, guy wiresfor tall buildings, bridge suspensions etc. jacket 603 may have dimplesto damp oscillations. The streamlined cable can be manufactured fromconventional round fiber rope by flattening it by pressure from thesides, using the flattened rope 601 as the force bearing core, and thenwrapping jacket 603 around it, with optional foam 602 inside of jacket603.

Examples of using streamlined cable in this invention are for cables 102in FIG. 1 and FIG. 4, cable 509 in FIG. 5, suspension cables in the kitewing in FIG. 3.

Multiple embodiments and aspects of the invention are described withreference to ground level. The invention can be practiced in marineenvironment (oceans, seas, lakes as well), in which case water levelreplaces ground level.

Thus, an airborne wind energy system with enhanced power transfer isdescribed in conjunction with multiple specific embodiments. While theabove description contains many specificities, these should not beconstrued as limitations on the scope, but rather as exemplification ofseveral embodiments thereof. Many other variations are possible.

What is claimed is:
 1. A device for conversion of wind energy intoelectric energy, comprising: an airborne wing, adapted to move in theair under power of wind; a round or aerodynamically streamlined cable,coupled to the airborne wing by one of the cable's ends; a perforatedbelt, attached to another end of the cable; a ground level platform; anelectrical generator, installed on the platform; a sprocket,rotationally coupled to a rotor of the electrical generator, thesprocket adapted to be engaged by the perforated belt.
 2. The device ofclaim 1, further comprising a drum for the perforated belt.
 3. Thedevice of claim 2, wherein the sprocket has smaller diameter than thebelt drum, thereby achieving higher angular speed of the sprocket. 4.The device of claim 1, further comprising an electronic control system,including a first control element installed on the platform, a secondcontrol element, installed on the wing, and a network link between them.5. A method of converting wind power into electric power, comprising:providing an electrical generator on the ground; harvesting wind powerusing an airborne wing with an attached cable, the cable having astreamlined or round section, the airborne wing pulling the attachedcable while flying mostly cross wind; converting the pull of the cableinto a linear motion of a perforated belt by connecting a free end ofthe belt to the cable; converting the linear motion of the perforatedbelt into rotational motion of a sprocket by engaging the sprocket bythe perforated belt; converting the power of the rotational motion ofthe sprocket into electrical power using the electrical generatorthrough rotational coupling between the sprocket and a rotor of theelectrical generator.
 6. The method of claim 5, further comprising astep of providing a drum for the perforated belt and further comprisingtwo alternating operational phases: the first phase comprising theairborne wing moving away from the sprocket, the perforated belt reelingoff the drum and electrical power being generated by the electricalgenerator; the second phase comprising the airborne wing moving towardthe sprocket, the perforated belt reeling on the drum and electricalpower being consumed; wherein significantly more electrical energy isgenerated in the first phase than consumed in the second phase.
 7. Themethod of claim 5, further comprising the step of utilizing anelectronic control system to control the electrical generator, the drum,the motion of the airborne wing and to synchronize reel on/reel off ofthe belt with the motion of the airborne wing.